In at least some circles, the term structured ASIC (application specific integrated circuit) has come to mean any ASIC that has at least one fixed layer of metal. Typically, all layers from contact down are also fixed. One difference between structured ASIC's and traditional gate arrays is that, with a structured ASIC, users can pay for fewer custom masks for their ASIC's, and more of the fixed masks can be shared between end-user designs.
Traditionally, creating an ASIC involves several steps, including synthesis, placement, and routing. A traditional ASIC router is capable of reading either LEF (library electronic format) files and DEF (design electronic format) files (a Cadence Design Systems, Inc. specific format), or GDSII (graphical design system 2). These files describe where components have been placed, and which connections the router needs to make to complete circuits. These connections are traditionally made on the configurable layers of metal by laying down metal traces on the various configurable metal layers, and vias between adjacent layers of metal to connect traces on different layers.
Traditionally, an ASIC router can use any configurable layer of metal, and route anywhere there is not already metal, so long as design rules (DRC or design rule checker) rules are met. DRC rules typically define minimum distances between isolated conductors on the various metal layers, and minimum width of these conductors, and often other DRC rules specific to the wafer fabrication process. Fixed metal typically occurs on the configurable layers for various reasons. For example, power trace routing is done as a pre-process to routing described above, and the power conductors are already defined on the configurable layers before the router runs as described above.
While many structured ASIC's are designed to reduce the number of configurable layers used in routing (thus reducing achievable circuit density), some structured ASIC fabrics are designed to use predefined fixed metal traces on the fixed layers of metal to conduct signals between the user's components. Using fixed metal in this way can improve circuit density. For example, a device disclosed in U.S. Pat. No. 6,580,289 makes use of wires on fixed metal layers to connect components, but the entire device can be configured using a single via layer.
Because traditional ASIC routers do not extract traces from the fixed layers of metal and make use of them in routing, techniques are used that rely on manual intervention, such as the use of customized scripts or even the manual creation of routing graphs. These approaches can lead to mismatches between the routing graph and the actual device, and resulting DRC rule errors. Additionally, the custom development effort increases the cost of the process.